1. Field of the Invention
The present invention relates to a surface property measuring apparatus and a method for controlling the same apparatus.
2. Description of the Related Art
There is known a surface property measuring apparatus for measuring a property (roughness, waviness or the like) of a surface of an object to be measured by running a stylus so as to trace the surface of the measured object. For example, a surface property measuring apparatus disclosed in JP-A-2012-225742 includes a measuring arm which is supported so as to make an arc-like movement, a stylus provided at a distal end of the measuring arm, a measuring force application unit which applies a force to the measuring arm so that the stylus is brought into abutment with the measured object with a predetermined measuring force, a moving mechanism for moving the measuring arm relative to a stage, and a displacement detector which detects a displacement of the measuring arm as a result of the measuring arm having made an arc-like movement.
FIG. 9 shows the measuring arm of the surface property measuring apparatus. An upwardly directed stylus 11 and a downwardly directed stylus 12 are provided at a distal end portion (at a left end portion in FIG. 9) of the measuring arm 10 so as to project therefrom perpendicularly upwardly and downwardly, respectively, relative to a longitudinal direction of the measuring arm 10. The measuring arm 10 is supported so as to rotate about a rotating shaft 13.
A voice coil motor 20 is provided at a proximal end portion (a right end portion in FIG. 9) of the measuring arm 10. When the voice coil motor 20 is energized, the measuring arm 10 is biased in an arc direction (here, either of upward and downward directions) about the rotating shaft 13. (In this case, the voice coil motor 20 constitutes a measuring force application unit.)
FIGS. 10A and 10B show a way in which the measuring arm 10 traces a surface to be measured of an object to be measured, or a measured surface S of a measured object W to measure a property thereof.
In the case of the measured surface S lying below the measuring arm 10, a distal end of the measuring arm 10 is biased downwardly so that a distal end of the stylus 12 is brought into contact with the measured surface S and further, a certain measuring force is generated between the measured surface S and the stylus 12 (an arrow MD in FIGS. 10A and 10B).
On the contrary, in the case of the measured surface S lying above the measuring arm 10, the distal end of the measuring arm 10 is biased upwardly.
Here, when the measuring arm 10 moves to follow the measured surface S while tracing it, the measuring arm 10 has a limit angle at which it can follow the measured surface S. This limit angle at which the measuring arm 10 is allowed to follow the measured surface S will be referred to as a following limit angle.
FIG. 10A is a drawing showing a case where an inclination angle of the measured surface S is moderate. In this case, since the inclination angle of the measured surface is equal to or less than the following limit angle, the stylus 12 is allowed to make a tracing movement along the measured surface S while allowing the stylus 12 to be kept in contact with the measured surface S with a constant measuring force.
FIG. 10B is a drawing showing a case where the inclination angle of the measured surface S exceeds the following limit angle. In this case, since the inclination angle of the measured surface S exceeds the following limit angle, the measuring arm 10 cannot follow the measured surface S. Then, the stylus 12 moves away from the measured surface S to lose the contact therewith and is caused to float in the air above the measured surface S momentarily. However, as this occurs, a downward biasing force which results from a sum of a weight of the measuring arm 10 itself and a weight of the voice coil motor 20 is applied to the distal end of the measuring arm 10. Consequently, this downward biasing force rotates forcibly the measuring arm 10 as the distal end of the measuring arm 10 falls as it were.
Hereinafter, the fact that the measuring arm 10 rotates forcibly as the distal end of the measuring arm 10 falls as it were may be expressed from time to time as the “measuring arm falls.”
There may exist a situation in which the stylus 12 loses the contact with the measured surface S to thereby float in the air above the measured surface S momentarily, whereafter the measuring arm 10 falls forcibly, which brings the stylus 12 into collision with the measured surface S. This causes a problem that the stylus 12 and the measured object W are damaged.